Steamflooding is an enhanced oil recovery method in which saturated or superheated steam is injected into an oil-bearing formation to heat the oil to reduce its viscosity so it will separate from the oil sand and drain into the wellbore. The water from the cooled and condensed steam is pumped out of the well with the oil and is separated at the surface.
Multi-spot patterns are commonly used in steamflooding. By "multi-spot pattern," we mean an areal configuration featuring an injection well and more than one production well that are used for recovery of oil. Examples of multi-spot patterns are 4-spot, 5-spot, inverted 7-spot, and inverted 9-spot patterns. In five-spot pattern, four production wells are located in a square pattern with the injection well in the center, a layout similar to a five-of-spades playing card.
These patterns have also been used for dipping reservoirs, while ignoring the effect of dip on steamflood performance. By "dip," we mean the angle that a geological stratum makes with a horizontal plane (the horizon); the inclination downward or upward of a stratum or bed. A five-spot pattern is more commonly used for steamflooding dipping reservoirs because it becomes a middle-staggered line drive if one side of the pattern is aligned with the direction of dip. By "dipping reservoir," we mean a reservoir that intersects a horizontal plane at an angle greater than 5 degrees. By "steeply dipping reservoir," we mean a reservoir that intersects a horizontal plane at an angle greater than 10 degrees.
One approach for steamflooding steeply dipping reservoirs is disclosed by Yick-Mow Shum in U.S. Pat. No. 4,260,018, entitled "Method for steam injection in steeply dipping formations," which is hereby incorporated by reference for all purposes. In that approach, steam breakthrough at the updip outcrop of a steeply dipping heavy oil reservoir is prevented by the injection of a hot water bank above the point at which the steam is injected into the heavy oil reservoir.
Another approach is disclosed by Stewart Haynes, Jr. et al. in U.S. Pat. No. 4,434,851, entitled "Method for steam injection in steeply dipping formations," which is hereby incorporated by reference for all purposes. In that approach, steam is injected in a lower portion of the reservoir and cold water is injected in an updip portion of the reservoir.
A third approach is disclosed by Bassem R. Alameddine in U.S. Pat. No. 4,627,493, entitled "Steamflood recovery method for an oil-bearing reservoir in a dipping subterranean formation," which is hereby incorporated by reference for all purposes. In that approach, steam injection wells are located up-dip and down-dip of each oil-bearing reservoir. Some time after steam breakthrough in the upper-most one of the production wells, this well is converted to a steam injection well, and the original up-dip steam injection well is shut in. Some time after steam breakthrough in the lower-most one of the production wells, this well is converted to a steam injection well, and the original down-dip steam injection well is shut-in. Some time after steam breakthrough occurs at the remaining up-dip and down-dip production wells, these wells are sequentially converted to steam injection wells, and the preceding up-dip and down-dip steam injection wells are shut in.
A recent simulation study of steamflooding in a steeply dipping reservoir has shown that, because of gravity, the injected steam becomes unevenly distributed between the updip and downdip parts of the reservoir. (K. C. Hong, "Effects of Gas Cap and Edgewater on Oil Recovery by Steamflooding in a Steeply Dipping Reservoir," SPE 20021,1990) Steam preferentially flows updip, causing early steam breakthrough to the updip producer while the downdip producer remains cold. This imbalance of steam flow produces poor areal and vertical sweep by steam and reduces steamflood efficiency.